The present invention refers to a device or means for carrying a friction pad, which is a part of a system for retarding rotational movement, e.g. at a rotating shaft, said carrier means being intended for distributing the press force from a brake mechanism via a contact surface to the friction pad mounted upon said means, which in its active position cooperates with a surface connected to the object of rotation wherein the carrier means with the friction pad is oriented extending along a sector of the rotation diameter of the contact surface.
Sometimes, a so called brake squeal occurs when a vehicle is retarded or braked, a phenomenon which occurs both in disc and drum brakes. Typical frequencies for this noise lies in the area of some kHz, and within the sensitivity range of the human hearing. It is true that the noise which is generated decreases with the distance from the source of the noise, but the noise may amount to about 140 dB close to the brake. Especially disturbing are town busses stopping at short intervals within densely populated regions. Brake squeal is a common cause of complaint regarding new cars, trucks and busses. Even if brake squeal does not affect the efficiency of the brakes and the security, the problem is regardless very large and has to be solved. The problem also occurs on vehicles having so called ABS-brakes.
Many analyses of the problem of brake squeal has been made, and many solutions have been tested. For example, tests have been made where the components of a brake system have been enlarged in order to get a different natural oscillation.
This has led to increasing costs, without getting any guarantee for not getting the brake squeal phenomenon under certain circumstances.
Further, GB 2143916 shows a disc brake pad with a back plate for a friction pad which varies in thickness along a line in the direction of rotation. This means that the thickness of the pads varies in the direction of rotation, which leads to negative consequences. For example, the life expectancy is reduced because the entire pad volume may not be used. Besides, the wear will be larger on one side, which will be the cause of local increases in temperature with thermal stress, which in the worst case will cause formation of cracks in the drum or the disc, perhaps leading to breakdown. Further, an unfavourable distribution of friction between different brakes may lead to the brakes pulling sideways, i.e. a brake fault which affects the steering. Also, there is a risk for shearing off the brake pad, which may lead to a total loss of braking power or to locking of some wheel.
It is obvious that no measure which is intended to reduce the problem of brake squeal, may lead to a reduction of break efficiency.
One object of the present invention is therefore to provide a carrier means for a friction pad, which in a simple and efficient way eliminates the problem of brake squeal, without decreasing the efficiency of the brakes.
For this object, the invention is characterized in that the carrier device is so designed that the value of the local natural angle frequency varies between a front end of said sector portion of the carrier device delineated along the rotation diameter of the contact surface and a back end of said sector portion of the carrier device.
This can be achieved in that either the back plate or the spar web, or both these elements are designed for varied natural angle frequency.
Advantageous variants for providing such variations are illustrated by the following subclaims.
The background to the expression local natural angle frequency.
Instable oscillations (self-oscillation) are by nature wave motion. Wave velocity for bending waves are proportional to the square root of the quotient between the bending stiffness, EI(xcfx86) and the distribution of mass, m(xcfx86), according to:   v  ~                    EI        ⁡                  (          ϕ          )                            m        ⁡                  (          ϕ          )                    
The natural angle frequency is proportional to the square root of the quotient between the bending stiffness, EI, and the mass, M according to:       ω    _    ~            EI      M      
Let us therefore define xe2x80x9clocal natural angle frequencyxe2x80x9d, xcfx891(xcfx86)                     ω        _            1        ⁡          (      ϕ      )        =                    EI        ⁡                  (          ϕ          )                            m        ⁡                  (          ϕ          )                    
so that
vxcx9cxcfx891(xcfx86)xc2x7
The distribution of mass, m(xcfx86), has the unit kg/m and is the product of cross sectional area, A, and density, xcex6, according to
m(xcfx86)=A(xcfx86)xc2x7xcex6(xcfx86)
i.e.
m(xcfx86)xcx9cA(xcfx86)xc2x7
The history of a brake pad (that is to say the wear that the brake pad has been subjected to during its life time) has a large significance upon the distribution of pressure which the brake pad exerts upon the drum. Also brake pressure and generation of heat during braking has a great influence upon the distribution of pressure. The different distributions of pressure makes different parts of the brake shoe active at different times. If the pressure is large at the sides of the shoe, the carrier plate for the pad, the back plate, is more active than the spar web. If the pressure is substantially located to the middle of the brake shoe, the spar web is more active than the back plate. Accordingly, the natural angle frequency is varied in the carrier device by special design of either the back plate or the spar web, or by designing these two elements for varied natural angle frequency.
Analysis of drum brakes has also shown that unstable oscillations in that area of frequency where brake squeal occurs must contain wave movement. If this wave movement is prevented, instability is reduced. It is known that a part of a wave is reflected when transiting into local natural angle frequency. When a wave is reflected in this way, a standing wave is created together with a reduced moving wave before the point of reflection and a reduced moving wave after the point of reflection. Thus, the reflection results in a decreasing moving wave. Because of this, instability is also reduced.
Transitions in local natural angle frequency may for example be provided by step-by-step alterations. One step is here defined by that the local natural angle frequency for the section cuts the local natural angle frequency mean value for the entire section portion of the carrier means, in such a way, that the received step lengths, 1, meet the condition 1 greater than 3t where t is the mean thickness value of the spar web. 
If the local natural angle frequency of the carrier means is not altered in the form of a step, the maximum-/minimum values shall be used within each step length according to the definition.
Both during continual and step-by-step alteration between two adjacent steps is applicable:                     ω        _            i                      ω        _                    i        +        1               less than       q    ⁢          xe2x80x83        ⁢    or    ⁢          xe2x80x83        ⁢                            ω          _                i                              ω          _                          i          +          1                       greater than       1    q  
where xcfx89i and ixcfx89l are two adjacent local natural angle frequencies or two local natural angle end frequencies.
The brake shoe is divided into the segments A=back plate, B=spar web and AB=the total section.
For these segments shall be applicable:
qA less than 0,7qB less than 0,7qAB less than 0,8 